1. Technical Field
This disclosure relates generally to thin-film magnetoresistive read sensors and particularly to the enhancement of the magnetic stability of such sensors.
2. Description
In its simplest form, the usual giant magnetoresistive (GMR) read sensor consists of two magnetic layers, formed vertically above each other in a parallel planar configuration and separated by a conducting, but non-magnetic, spacer layer. Each magnetic layer is given a unidirectional magnetic moment within its plane and the relative orientations of the two planar magnetic moments determines the electrical resistance that is experienced by a current that passes from magnetic layer to magnetic layer through the spacer layer. The physical basis for the GMR effect is the fact that the conduction electrons are spin polarized by interaction with the magnetic moments of the magnetized layers. This polarization, in turn, affects their scattering properties within the layers and, consequently, results in changes in the resistance of the layered configuration. In effect, the configuration is a variable resistor that is controlled by the angle between the magnetizations. If the sensor is constructed and magnetized so that one of its magnetic layers, the “pinned layer” has its magnetic moment fixed in spatial direction by an adjacent “pinning layer,” while the other magnetic layer, the “free layer” has a magnetic moment that is unconstrained, it is called a spin-valve sensor.
The magnetic tunneling junction device (TMR device) is an alternative form of GMR sensor in which the relative orientation of the magnetic moments in the upper and lower magnetized layers controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those magnetized layers. When injected electrons pass through the upper layer, as in the GMR device, they are spin polarized by interaction with the magnetization direction (direction of its magnetic moment) of that layer. The probability of such an electron then tunneling through the intervening tunneling barrier layer into the lower magnetic layer then depends on the availability of states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower layer. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability multiplied by the number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer. The TMR sensor is also typically formed in a spin-valve configuration, comprising a free layer, and a pinned/pinning layer structure.
When the TMR configuration is used as a sensor or read head, (called a TMR read head, or “tunneling magnetoresistive” read head) the free layer magnetization is required to move about a central bias position by the influence of the external magnetic fields of a recorded medium, such as is produced by a moving hard disk or tape. As the free layer magnetization varies in direction, a sense current passing between the upper and lower electrodes and tunneling through the dielectric barrier layer varies in magnitude as more or less electron states become available. Thus a varying voltage appears across the electrodes (which may be the magnetic layers themselves). This voltage, in turn, is interpreted by external circuitry and converted into a representation of the information stored in the medium.
To increase the area storage density of a hard disk drive (HDD) system, trackwidth reduction in both the reader and writer sensor elements is required. With a reduction in the reader trackwidth, a reduction in its height is also required, this height being essentially the thickness of the sensor strip that forms the read element. Thus, the total volume of the read sensor must be reduced if its trackwidth is to be reduced for increased reading resolution and area storage density of the recording medium.
It is also well known that the magnetic stability of the read sensor is proportional to the device volume. Thus, as the reader sensor dimensions shrink, magnetic noise associated with the pinned and pinning layers becomes an increasing problem. This adversely impacts the reader sensor performance, as has been discussed by each of the following: A. Ozby et al., “Low frequency magnetoresistive noise in spin-valve structures,” Appl. Phys. Lett., 94, 202506 (2009); A. Akimoto et al. “Analysis of thermal magnetic noise in spin-valve GMR heads by using micromagnetic simulation,” J. Appl. Phys., 97, 10N705 (2005); Y. Zhou, “Thermally excited magnetic noise from pinned and reference layers in current perpendicular-to-plane structure magnetoresistive heads,” J. Appl. Phys. 103, 07F516 (2008).
The prior arts show attempts at resolving performance problems stemming from the shape of sensor magnetic layers. Examples are: Xue et al., (U.S. Pat. No. 8,184,409 and U.S. Pat. No. 7,835,116); Watanabe et al. (US Pat. Appl. No. 2007/0206333) and Yasui et al. (U.S. Pat. No. 8,223,464). However, none of these attempts have addressed the problem of sensor volume reduction in the same manner and with the same effect as the method to be summarized below and then described in further detail herein.